How are you going to refill the bottle? If it's built in with a fill port then I'd go bigger. If it's portable then smaller is easier to deal with. Also remember that FBOs will often charge a flat rate for an 02 refill, so a larger bottle gets you more for your $$$.

I've found that I need to fly 300+ mile legs to make it worth while to climb high enough to use 02. Anything shorter doesn't make it worth the climb.

This is a portable system. I am looking at flying from DFW area to Eugene OR and thought the O2 would help over the Rockies and help my pax avoid headache. The 22 cf would be easier to stow but may need refilling on the road.

If you use oxygen conserving cannulas 22 cubic feet will last a long time. Get a pulse oxymizer while you're at it.
I use the E size cylinder. Fits nicely between the seats, on top of the center tunnel.
PS My wife used to get headaches above 10k'. Just a very small flow of O2 and no headaches. I feel better, too.

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